M80 BASS combo, anybody want to help a dufus??

[Warped]

M80 Bass. Yesterday at our blues jam (at my home) it started making a cracking noise then it got worse so that it sounded like you're funning through a fuzz box.
Today I hooked it up to one of the PA cabs (eliminating the combo speaker) and it still did it. Distorted like crazy.
I tried using the other input, turning on or off the chorus and Deltacomp (I never use those usually). All the pots are quiet and smooth.
I took it apart and checked for anything loose. I made sure everything was secure. Desoldered and resoldered a couple joints on the preamp board. With the amp sitting on top of the cabinet it sounded fine. Installed it in the cabinet and it's back. This doesn't sound like a cabinet or plywood problem, it sounds electrical.
If you guys could give me some ideas on things I could check I'd be very grateful. I don't know much about the innards of electronics. I have decent soldering skills and a multimeter (that I have no clue how to use other than checking batteries). The local shop will not look anything without a schematic and are more of the mindset of "just buy a new one, it would cost more than it's worth to look at it." This is a real nice amp for what we use it for.
The vibration of the cabinet is making something go wacky, but it's hard to figure out because when the chassis is pulled out it works fine. Thanks for reading anyway.

[Enzo]

Ball up your fist and whack it. Does it react? it shouldn;t. or use a rubber mallet if you want to be fancy.

With it running and a speaker connected, go over the board prodding each part with something insulated, like a wooden chopstick. Anything reacts is suspect.

Recheck the solder on larger components like filter caps and the larger rectangular cement power resistors. Check the solder all along the panel on jacks and controls.

As to the local shop... good lord. How hard is it to come up with a Fender schematic?

[Warped]

Thanks Enzo, I do that tonight. I was wondering how to replicate the vibration with the chassis sitting on the bench. I'm sure this will help a lot.

[Warped]

I'm still not able to find the problem with my M80 Bass amp. Does anyone happen to know if the light beside the Delta-Comp switch is supposed to come on if the switch is disengaged? This amp had a problem a couple years ago where it would make a squealing noise when the Delta-Comp was turned on. It stopped doing that but I am a little suspicious of it. I'd like to bypass it if I could with the compression turned off, it never really gets used anyway. I tried tapping away on the components with the old chopstick while my wife was playing notes on the bass for me. Now it is buzzing/distorting while on the bench with the cabinet a ways away. It stopped for a little while but started up again on it's own. Pretty confusing.

[Warped]

I guess it's time to try to find a shop that will look at it.

[Twist]

Some cracked or bad solder joints are hard to see without a bright bench light. As for the shop who has to have a schematic for everything, I would go broke and have to close shop if I turned away work because I didn't have a schematic.

[Enzo]

Amen.


Have you checked the power supplies? Make sure the main filter caps have good solder. Check that both positive and negative main voltages are about the same and virtually free of ripple on the power amp. Also check the low voltage for the preamp. +/-15vDC are both of those up to voltage and free of ripple?

[nbtone]

All bass amps have problems like this after sitting around for a while. What you need to do is clean your pots and plug-in connectors. Anything that isn't soldered has to be taken apart and cleaned. I just periodically spray everything with wd-40 and put it all back together. You have to be very careful about getting everything back together the way it came apart. Chips are liable to be destroyed if you don't ground yourself when you touch them, too.
You also need to check your guitar chord for shorts. Clean the pots and plug-in connectors in your bass, too.
It might be best to take it to a shop, where a pro can take care of it for you. A lot can go wrong.